Spring is coming earlier, and nature is scrambling to keep up, according to scientists who say climate change is to blame.

The season starts an average of 10 days earlier in the United States than it did just 20 years ago. And that is scrambling the delicate balance of many ecosystems, as some species adapt to the change and others don't.

"What's happening is that the Earth is warming," said Jake Weltzin, director of the National Phenology Network, a federal program that tracks changes in seasonal patterns. "That is causing changes in when things start in the spring. But what happens, though, is not all plants and animals respond the same."

For reasons scientists don't entirely understand, the climate mismatch appears to often favor invasive species over native species.

That's what's happening at one of the United States' most iconic landscapes -- Walden Pond in Concord, Mass., the site documented by Henry David Thoreau.

Climate change has altered the plants found in the woods near the pond, according to Charles Davis, an evolutionary biologist at Harvard University. He has studied plant life there by juxtaposing modern observations with historical records kept by Thoreau and other botanists.

"It turns out that the losers, by and large, tended to be our native plant species," he said. "What this study documents is that climate change is not affecting species uniformly. There are certain groups that are being hit much harder than others."

Walden Pond's native plants -- including lilies, orchids, roses and dogwoods -- seem to have a harder time adjusting their flowering to match the earlier emergence of spring.

Thirty percent of the plants once found at the pond are now extinct, and another 30 percent are present in such low numbers their extinction seems likely.

"The winners, by and large, are the non-native plant species," Davis said.

In the West, the shift to an earlier spring is affecting wildfire patterns. In areas like the mid-elevations of the northern Rocky Mountains, where spring temperatures are just under freezing in an average year, "it doesn't take a large increase in temperature to start melting snow earlier in spring," said Anthony Westerling, a professor of environmental engineering and geography at the University of California, Merced.

Lengthening fire seasons and aborted foals
That means longer snow-free summers that lead to longer fire seasons and drier trees.

"In the northern Rockies, there's been an increase in the number of fires and the area burned well exceeding 1,000 percent since the 1970s and early 1980s," Westerling said. "These fires all occur in years with early spring."

But the effects of the shifting spring are often nonintuitive, scientists said.

For the American pika, found in mountain areas in 10 Western states, "spring creep" means not just hotter summers, but colder winters in some areas, said wildlife biologist Erik Beever.

Beever, who has studied pika population data going back to 1898, said pika populations have shrunk in places that were warmest in summer.

"The interesting thing ... is the sites they were lost from were warmed in summer, colder in winter, because they lose their insulating blanket of snow," he said.

Scientists eventually figured out that the warm spring that year started the chain that led to the death of the foals. The unusual weather encouraged the early emergence of Eastern tent caterpillars before the arrival of the migratory birds that normally eat them.

"The caterpillars fell on the ground and they were ingested by mares," Weltzin said. "They contained a chemical that caused the mares to abort. ... There are interesting, nonintuitive processes that can occur due to changes in the timing of spring."

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